Re: [PATCH v10 2/8] mm: rust: add vm_area_struct methods that require read access

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Dec 5, 2024 at 11:45 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Thu, Dec 05, 2024 at 11:40:03AM +0100, Alice Ryhl wrote:
> > On Fri, Nov 29, 2024 at 5:32 PM Alice Ryhl <aliceryhl@google.com> wrote:
> > >
> > > This adds a type called VmAreaRef which is used when referencing a vma
> > > that you have read access to. Here, read access means that you hold
> > > either the mmap read lock or the vma read lock (or stronger).
> > >
> > > Additionally, a vma_lookup method is added to the mmap read guard, which
> > > enables you to obtain a &VmAreaRef in safe Rust code.
> > >
> > > This patch only provides a way to lock the mmap read lock, but a
> > > follow-up patch also provides a way to just lock the vma read lock.
> > >
> > > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
> > > Reviewed-by: Jann Horn <jannh@google.com>
> > > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> >
> > It looks like binder needs a way to check whether a given vma is
> > associated with a given mm. I guess we can do that by adding a method
> > to VmAreaRef that returns a &MmWithUser.
>
> Presumably this would be with a lock held to ensure the VMA doesn't
> disappear from under us?
>
> I guess that's implied by possessing a VmAreaRef in the first place.
>
> I also suppoes that the mm having users is implied by you having the VMA
> that implies a lock held on it :)

That was what I was thinking.

> So that's probably fine then, as long as you can implement some sensible
> means of comparison between a known 'given mm' vs. the &MmWithUser.

We can use the stdlib function core::ptr::eq to compare the address.


Alice